One known blood constituent measuring device is a blood oximeter which is a photoelectric photometer utilized for measurement of the fraction of hemoglobin in blood which is in the form of oxygenated Hb, that fraction is normally expressed in a percentage referred to as saturated oxygen content. The basic principles of blood oximetry based on these techniques are discussed in an article entitled .cent.Oximetry" by Earl H. Wood, et. al., appearing at pages 416-455, Medical Physics, vol. 2, O. Glasser, Ed., Year Book Medical, Chicago, Ill (1960).
A number of oximetry devices and methods utilizing light in the red and infrared regions are shown in U.S. Pat. Nos. 3,638,640; 3,647,299; 3,704,706; 3,804,539; 3,998,550; 4,086,915 and 4,167,331.
Many previous oximetry devices use complex logarithmic functions to determine oxygen saturation of the blood as is shown, for example, in U.S. Pat. Nos. 3,638,140; 3,804,539; 3,998,550; and 4,167,331. The apparatus disclosed in U.S. Pat. No. 4,167,331 employs a digital microprocessor in conjunction with analog logarithmic amplifiers to calculate blood oxygen saturation. Other devices have utilized three frequencies, with synchronous detection, peak detection and ratio circuits, for example, that shown in U.S. Pat. No. 3,647,299.
Finally, there are blood oxygen saturation measurement devices employing digital techniques including computer microprocessors for calculating pulse rate, oxygen saturation and confidence factors for those values (see European Patent Application Publication No. 0104771) and one such device further includes normalization of the DC signal components from each light source and an AC modulated test signal for testing the device (see U.S. Pat. No. 4,407,290).
Nevertheless, even those devices employing digital techniques have not proven to be completely satisfactory in delivering reliable, blood constituent measurements particularly in the operating room environment. Therefore, a need exists for a compact, highly noise resistant, accurate and reliable blood constituent measurement apparatus and method.